140 UNIVERSITY OF COLORADO STUDIES 



the lighter weight and lesser cost of the former make them much 

 more desirable. Concrete and steel further seem to have very nearly 

 the same coefficient of expansion. Under varying temperatures the 

 combination may be expected to act as one. 



The design here considered will for the above reasons and for 

 one other given below be in concrete-steel. 



The methods for finding the stresses in fixed ended arches are 

 the same for different materials but in proportioning the rib sections 

 a modification of the formulas of flexure is necessary in cases where 

 concrete-steel is employed. 



During the winter of 1899-1900 the writer had the good fortune 

 to be associated with Prof. Wm. H. Burr, of Columbia University, 

 New York, and at that time assisted him in preparing designs for a 

 proposed memorial bridge across the Potomac River at "Washington, 

 D. C. Three different designs were prepared and all contained con- 

 crete-steel arches. Some of these arches were semi -circular, some 

 circular-segmental, and others semi -elliptical. Their spans ranged 

 from 40 to 283 feet. 



In the following design the 192-foot main arch span of Design 

 No. II for the Memorial Bridge will be considered. 



This arch has a circular-segmental intrados, radius 173.4 feet, 

 the clear span at the springing line being 192 feet, and the rise of 

 the intrados 29 feet. The extrados is also an arc of a circle but is 

 not concentric with the intrados. Its radius is 213.8 feet. The as- 

 sumed depth of rib at the crown is 40 inches, that of the springing 

 lines is 100 inches. The center line or neutral curve of the rib is an 

 arc of a circle and is drawn midway between the intrados and extra- 

 dos. Its radius is 192.0 feet. 



The ribs are of concrete in which are embedded masses of steel 

 near both the intrados and the extrados. From sections 1 to 18, 

 throughout the main part of the rib, bars of steel S"x^" spaced 2 ' 8" 

 center to center are used in both the top and bottom of the concrete 

 mass. In no case does the steel come nearer to the surface of the 

 concrete than 2 inches. At the springing points and near the ends 

 of the rib heavier steel work is used on account of the higher tensile 



